1. Field of the Invention
The present invention generally relates to dipoles and, more particularly, to a wideband -shaped monopole dipole.
2. Description of Related Art
For base station systems in mobile communication, wideband antennas which have high front-back ratio and medium beamwidth are generally needed, for instance wideband antennas having beamwidth of 90°, 65° or 32°, or wideband antennas used in a dual-band common use of CDMA and GSM, DCS and PCS, or PCS and UMTS, or wideband antennas used in a tri-band common use of DCS, PCS and UMTS. Known wideband antennas which can meet requirements of the above radiation characteristics are corner reflector antennas and logarithm periodic antennas. However, corner reflector antennas and logarithm periodic antennas usually have large size and high manufacturing cost.
To overcome the shortcomings as previously described, persons in the field of mobile communication analyzed and found that:
A half-wave symmetrical dipole having a beamwidth of about 75˜80° in an H-plane is set above a boundless metallic-based reflective plate and has a quarter-wave distance to the boundless metallic-based reflective plate. When the height of the dipole is adjusted, the beamwidth will also change accordingly. However, the change range of the beamwidth is narrow and the beamwidth of 90±8° or 65±6° is unlikely to be obtained. If the reflective plate is limited in a range of less than four-fifth wave, the front-back ratio of the dipole is unlikely to be controlled below the range of 28 dB to 30 dB.
To obtain a predetermined beamwidth in the H-plane and a high front-back ratio, based on suggestions of double-ring antennas, a -shaped monopole dipole is formed by a connection of two symmetrical dipoles in a same plane by a parallel-wire, and a balance feed on a center portion of the parallel-wire. Via adjusting the size of the symmetrical dipoles and the impedance of the parallel-wire, an impedance match on feed points can be performed.
As shown in FIG. 1, the improved -shaped monopole dipole has a simpler configuration and fewer feed points and, thus, control of the beamwidth in the H-plane and the front-back ratio is able to be realized, which can overcome shortcomings of the corner reflector antennas and the logarithm periodic antennas. However, in practice, since the bandwidth with the standing wave ratio thereof less than 1.30 is about 10%, the -shaped monopole dipole can only meet the requirements of a single-band of CDMA, GSM, DCS or UMTS, but cannot meet the requirements of a dual-band common use of CDMA and GSM824˜960 MHz, a tri-band common use of DCS1710˜1880 MHz, PCS1850˜1990 MHz and UMTS1920˜2170 MHz, or a dual-band common use of two of DCS1710˜1880 MHz, PCS1850˜1990 MHz and UMTS1920˜2170 MHz.
In summary, employing the wideband -shaped monopole dipoles as radiation units is simple and reliable, and has a lower cost manufacturing. However, if the metallic-based reflective plate is limited, the front-back ratio is hard to control. Also, the ordinary conventional half-wave dipoles cannot perform a wideband capability. When a number of radiation rows are utilized to control the front-back ratio, a complex feed network is needed, which will inevitably increase the manufacturing cost.